Metal: Mining from metal mines. (These occasionally sparkle, to make them easier to tell apart from stone mines.)

(On certain maps, you can also quickly gather a great deal of resources from special treasures found on the map, such as shipwrecks.)

Shuttling to Dropsites

When units gather a resource, there is only so much they can carry. Then they need to shuttle the resources they have gathered to a building that serves as their resource dropsite and drop it off there. Only then can they return to gather more resources.

Civic centers and docks serve as dropsites for all four resources.

Mills serve as dropsites for wood, stone and metal only.

Farmsteads serve as dropsites for food.

Units automatically shuttle the resources they have been gathering to the nearest dropsite that can accept it. It might be a good idea to build dropsites near their corresponding resources, to make sure your gathering units only have to shuttle short distances, which makes gathering more efficient.